When a focussed electron beam strikes a specimen, signals are generated that are representative of the material excited by incident electrons. In a scanning electron microscope, the position of such a focussed electron beam on the specimen surface can be controlled by deflecting the beam using electrostatic or electromagnetic methods. The analysis control system will typically position the beam at a series of points on a grid. At each point one or more signals are measured and the values obtained for each type of signal corresponding to that point are recorded. The values for a signal obtained on a regular rectangular grid form a digital image from a field of view. The electron beam can be deflected very fast but the magnitude of the deflection is limited by the electron optics because there is increasing distortion at large deflection angles. Therefore the field of view may only cover a small area on the specimen surface. If a large area of the specimen surface has to be covered, a new field of view can be brought under the beam by moving the specimen stage that supports the specimen. Thus, by a combination of stage movements and beam deflections, the whole surface of the specimen may be surveyed in a series of fields of view. The components of an example apparatus for automated particle analysis are show in FIG. 1.
The time to scan a grid of points over the surface of a specimen depends on the number of points in the grid and the dwell time at each point. In a scanning electron microscope, electron signals usually have much better signal-to-noise than other analytical signals such as x-ray spectral emissions. This means that useful electron signal information can be obtained with a much shorter dwell per point than for x-ray or other analytical data. It is well known that a fast electron signal scan over the specimen can be used to find out if there are any features (e.g. particles, debris, defects etc.) that make it worthwhile spending additional time for x-ray analysis (e.g. Laskin and Cowin, Anal. Chem. 2001, 73, 1023-1029). This principle is used for finding contaminant particles in precision manufacturing, also called ‘technical cleanliness analysis’. Particles are removed from the manufactured part and deposited on a substrate made out of a light element such as carbon. The signal from a backscattered electron detector (BSED) is sensitive to the mean atomic number of the material and if the electron beam strikes a contaminant particle containing heavy elements, the signal will be strong. However, X-ray analysis is still required to determine which elements are present in the particle to identify the source of the contamination. By scanning the electron beam over a grid of points and observing the BSED signal, the location of potentially interesting particles can be found fast without wasting time doing slow x-ray analysis on every point. X-ray analysis time can thus be minimised by concentrating on the most likely particles and the overall search process is efficient.
The electron signal strength mapped over a regular grid of points forms a two dimensional digital image of pixels which take values corresponding to the electron signal at each pixel location. The image can be processed mathematically to identify features such as particles, measure the morphology of individual particles and count the number of particles. If the grid of points is sufficiently fine, then the measurements of particle morphology may be accurate enough to characterise particles without using any additional analytical signal. However, a fine grid means there are more points for the beam to dwell on to get a good electron signal and consequently a longer time to finish a single scan by deflecting the electron beam.
The conventional method of automated particle analysis is exemplified by FIG. 2. FIG. 2(a) shows a specimen where there are a number of particles on the surface. While the stage is stationary, the electron beam is deflected sequentially over a grid of points, typically with a raster scan order, in order to collect a digital image of the electron signal (e.g. backscattered electron signal, BSE) over a small field of view. This is shown schematically in FIG. 2(b). The specimen can be moved in X and Y directions by moving the stage supporting the specimen. Thus the position of the electron beam raster can be arranged to fall on contiguous fields of view that cover a large area on the specimen surface. In FIG. 2(c), 9 contiguous fields are shown labelled in the order in which they will be scanned. FIG. 2(d) shows the particles that will be covered by the raster scans in each field. When field 1 is scanned, the digital image formed using an electron signal (FIG. 2(e)) is processed to find the outlines of individual particles and decide if each particle is of a size and shape to be of interest. For each particle of interest, the electron beam is then either positioned on a point on the particle, or rastered over a grid of points on the particle while x-ray data is acquired (FIG. 2(f)). This is repeated for all the particles to be analysed in field 1. When all particles of interest have been analysed, the specimen stage is moved so that the next field of view can be reached by electron beam scanning. The process is repeated until all 9 fields have been covered.
The region that can be scanned by deflecting an electron beam is limited and a combination of specimen stage movement over a coarse grid and electron beam deflection on a finer grid in between the stage grid points is usually required to scan the whole specimen. The number of points in the electron beam grid and the number of points in the stage grid can be optimised to minimise distortion and deliver “target” data of required accuracy in the fastest possible time. Nevertheless, even if no additional analytical measurements are taken, it may take many stage movements and analysis of many grids of points of electron signal data in order to cover the whole specimen region. If there are no potentially interesting particles found after an electron signal scan, the time for the stage movement and the time to scan the electron beam over that grid of points is essentially wasted. In the worst case, the time for surveying the whole specimen may be wasted and this can be important if there are many specimens to be scanned. The object of the current invention is to improve the productivity for surveying samples.